1. Field of the Invention
This invention relates to a device for tempering of a glass sheet by contact to produce reinforced edge stresses, the sheet also optionally undergoing bending by such contact. The invention applies particularly to the production of glazings tempered thermally, which are flat or curved and intended, for example, for motor vehicles.
2. Background of the Related Art
Usually, the heat tempering of glazings is performed by blowing cold air on both faces of glass sheets previously heated to a suitable temperature. In some cases, however, it can be more advantageous to perform a so-called "tempering by contact" by placing the glazings in direct surface contact with compression plates cooled, for example, with water, i.e., plates of the compression plate or compression mold type. The direct contact with the compression and cooling plates makes it possible to prevent deformation or warping of the glazing which at times appear when the glazing is held only along its edges and/or on a single side during tempering.
The tempering by contact process also can be advantageous for the production of bent and tempered glazings of very thin glass. The thin glass cools very quickly and when, according to the conventional bending-tempering processes, the glass sheet first is bent before being transferred to the tempering station by blowing of cold air, the cooling of the glass during the transfer can be such that its temperature is then insufficient for a quality heat tempering. The use of compression plates consisting of molds whose shape corresponds to that desired for the glazing makes it possible to eliminate the transfer, since bending and tempering of the glazing is performed in a single operation.
From patent application EP-A 277 074, it is known to reduce the fragility of the edges in the glazings tempered by contact in the usual way, such as that of FR-A 1 580 305, by creating a temperature difference between the edges and the center of the glazing, particularly by blowing a cooling gas on the edges of the glazing. There, a device is described in which the peripheral surfaces of the glazings are cooled by air jets while the operation of tempering by contact continues. Still according to EP-A 277 074, these air jets are produced by tubular ramps placed parallel to the edge of the glass, outside of the cooling and compression plates. These ramps are provided with outlets having slots for the emission of air jets.
This blowing of cold air on the peripheral surfaces allows three configurations which can be considered for the compression and cooling plates and the glazing: (1) the glazing can be larger than the plates, so that its ends project beyond the plates; (2) it can be of the same size as the latter with a peripheral surface which is even with the plates; (3) it can be smaller than the plates so that the peripheral surface of the glazing to be cooled by the air is set back relative to the lateral delimitation surfaces of the cooling and compression plates. In the third case, a groove is formed whose base surface consists of the peripheral surface of the glazing.
Also known from patent application DE-A 1 771 790 is a device for tempering by contact with compression plates, in which the first plate consists of a rigid metal plate cooled with water and the other plate is formed from a housing closed by a heat-resistant fluid tight membrane, which can be fed by pressurized water and whose membrane surface establishes contact with the glazing.
In this case also, the compression plates are larger than the glazings. The cooling of the peripheral surfaces of the glazings are then performed by the fact that the glazings are beveled on the side which is applied against the membrane so that the cooled membrane can be applied to the beveled surface. Such a beveling of the edges of the glazings cannot be achieved, however, in the case of automobile glazings.
For various reasons, this technique often proves more economical since a single set of the cooling and compression plates are suitable for glazings of various sizes but of the same radius of curvature. On the other hand, it is easier to obtain reinforced edge stresses with cooling and compression plates larger than the glass sheet because then the intensity of the blowing on the edges can be fairly weak, while with plates smaller than the glass sheet the blowing must also temper the marginal zones of the glazing which are not covered by the cooling plates. It is advantageous for the tempering by contact to use devices comprising compression plates larger than the glazings to be tempered. The known devices, however, do not make it possible to achieve a satisfactory state of tension of the glazings, either in the immediate vicinity of their edges or over their entire peripheral surface.